There are many calls to help bring down Carbon Dioxide levels. But what are the effects of such an action?

My intuition of the earth as a slow-moving complex system tells me that it will settle into a "new state" in which new patterns of temperature and rain will establish for a very long time.

But a widespread opinion says that we will able to reverse the rise in temperatures and other effects like rainfall patterns if we reduce emissions. It seems a simpler explanation to sell and to believe in (most people, like me, are not trained in complex systems theory!).

These emissions change the balance of gases in the atmosphere, and
cause climate change, global warming and degrade the ozone layer. It
up to all of us to reduce our emissions to reverse these effects.

I’ve never heard knowledgeable people (any people, actually) claim that we are able to reverse the current trend completely. Could you please cite a source for this claim to show that it’s notable?
– Konrad RudolphJun 10 '12 at 11:35

I included a (mis)qoute of IPCC (reputed source on this matter) on HowStuffWorks (popular factual site). I could also include a few of the "About 12,500,000 results" when you google "reverse global warming"; reputable popular science sources (Discovery, Daily Mail, National Geographic on first page) mention it easily, without caveats. Wouldnt that be enough to spread the notion?
– Jesvin JoseJun 10 '12 at 12:12

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Just for clarification: is this question about "Global Warming" or "Climate Change"?
– Oliver_CJun 10 '12 at 12:28

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Where did the "over a few decades?" in the title come from? Is there a notable source for that? If we're talking tens of thousands of years, it's probably an easy yes. So timescale is crucial
– EnergyNumbersJun 10 '12 at 15:37

1 Answer
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Without geo-engineering solutions, a certain amount of warming is irreversible. Even if we stop emitting CO2 today, the energy is in the system and it will cause a certain amount of warming.

Dr. James Hansen of NASA Goddard has talked a lot about this. He often says that there is already warming "in the pipeline" that is irreversible.

In a 2005 paper in Science Magazine he says:

Earth's energy imbalance. We infer from the consistency of observed
and modeled planetary energy gains that the forcing still driving
climate change, i.e., the forcing not yet responded to, averaged ∼0.75
W/m2 in the past decade and was ∼0.85 ± 0.15 W/m2 in 2003.
This imbalance is consistent with the total forcing of ∼1.8 W/m2
relative to that in 1880 and climate sensitivity of ∼2/3°C per W/m2.
The observed 1880 to 2003 global warming is 0.6° to 0.7°C,
which is the full response to nearly 1 W/m2 of forcing. Of the 1.8
W/m2 forcing, 0.85 W/m2 remains, i.e., additional global warming of
0.85 × 0.67 ∼ 0.6°C is “in the pipeline” and will occur in the future even if atmospheric composition and other climate forcings remain
fixed at today's values.

There is inertia in the Earth System. The CO2 has been emitted, and the radiation balance is off. It will take a while for that warming to show up in the climate, but it will happen. The Earth System is out of equilibrium and is working towards getting to it, and it will be a warmer equilibrium.

However, this doesn't mean we can just forget about trying to reduce CO2 emissions because we're screwed anyway. This image from the IPCC AR4 shows the model predictions of warming for different CO2 emission scenarios. The yellow line represents if we could manage to get our atmospheric CO2 concentrations down to 2000 levels, and the red line represents a business as usual approach where we make no changes whatsoever.

You can see from this chart that the scenario we choose, and how much CO2 that we emit in the future, does make a difference in considering how much warming will occur. Reducing CO2 emissions is crucial for being able to mitigate the amount of warming that will occur, but at this point we've already locked in a certain amount of warming becuase of past CO2 emissions.

The orange line on the graph suggests that a constant CO2 concentration might lead to almost constant temperatures so presumably a reduced CO2 concentration might lead to reduced temperatures
– HenryJun 10 '12 at 22:01

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@Henry the IPCC AR4 was published in 2007 and the yellow/orange line shows how much temperature would rise from the year 2000 had emissions stopped in 2000. That shows a rise of about ~0.5°C which is not trivial, certainly not constant. Atmospheric CO2 drives a positive feedback cycle of warming that isn't linear.
– TacroyJun 10 '12 at 22:32

Tacroy: that does affect my point that a constant CO2 concentration corresponds to a close to constant temperature in the graph. In fact global temperature has been essentially flat since 2000
– HenryJun 11 '12 at 6:21

Where was Tacroy during the moderator elections!? These answers all kick serious woo!
– JasonRJun 11 '12 at 17:43